Hot gas engines of the Stirling type require a low temperature space as part of a closed gas system. The low temperature is achieved by way of a cooling system which typically employs a fan assembly for moving ambient air through a heat exchange or radiator unit. The operating demand placed upon a fan assembly for a reciprocating engine is not as severe as that for a hot gas engine where a greater flow of ambient air at higher velocities is necessary. At the same time, a hot gas engine of the Stirling type, must be extremely sensitive to undue weight increases. Accordingly, it is desirable that the weight of the fan assembly be minimized while at the same time insuring high efficiency. This is a difficult problem to overcome since the Stirling engine requires a larger radiator than the internal combustion engine; it is roughly estimated that a Stirling engine requires a radiator of about 2.5 times as large as that of an automotive engine for the same supplied horsepower. The fan requirements are multiplied accordingly.
To reduce weight and at the same time provide for ease of manufacture and higher efficiency, the prior art has attempted to stamp a plurality of fan blades from a common sheet of metal; the stamped sheet is assembled by fasteners to a heavy open-fingered type of hub. However, such method did not allow the fan blading to assume an air foil configuration but rather a flat configuration. The described standard method of blade to hub attachment limited the number of blades and degree of pitch angle that fan assemblies could accept. In the turbine wheel art, a similar approach with stamped sheet metal blading has been used; the blading was twisted at its neck into a predetermined air foil contour, but the twisting of the neck of several blades from a common hub flange created stress points which would not operate consistently over the intended life of the fan.